The molecules of a liquid crystal tend to align with one another in substantially one direction, which results in a fluid material having anisotropic optical, electromagnetic, and mechanical properties. A mesogen is typically described as the primary or fundamental unit (or segment or group) of a liquid crystal material that induces, and/or is induced into, structural order amongst and between liquid crystals (e.g., other liquid crystal materials that are present).
Liquid crystal polymers are polymers capable of forming regions of highly ordered structure while in a liquid phase. Liquid crystal polymers have a wide range of uses, including engineering plastics, and gels for LC displays. The structure of liquid crystal polymers is typically composed of densely packed fibrous polymer chains that provide self-reinforcement almost to the melting point of the polymer.
Dichroism can occur in liquid crystals due to the optical anisotropy of the molecular structure, or the presence of impurities, or the presence of dichroic dyes. As used herein, the term “dichroism,” and similar terms, such as “dichroic” means the ability to absorb one of two orthogonal plane polarized components of radiation (e.g., transmitted and/or reflected radiation) more strongly than the other orthogonal plane polarized component.
Linearly polarizing elements, such as linearly polarizing lenses for sunglasses and linearly polarizing filters, are typically formed from orientated, such as unilaterally orientated, polymer sheets containing a dichroic material, such as a static dichroic dye. Consequently, conventional linearly polarizing elements are static elements having a single, linearly polarizing state. Accordingly, when a conventional linearly polarizing element is exposed to either randomly polarized radiation or reflected radiation of the appropriate wavelength, some percentage of the radiation transmitted through the element is linearly polarized. As used herein the term “linearly polarized” means to confine or effectively limit the vibrations of the electromagnetic vector of light waves to one direction or plane.
In addition, conventional linearly polarizing elements are often tinted. For example, conventional linearly polarizing elements can contain a coloring agent, such as a static dichroic dye, and correspondingly have an absorption spectrum that does not vary in response to actinic radiation. The color of conventional linearly polarizing elements typically depends upon the coloring agent present in the element, and is often a neutral color (e.g., brown or gray). As such, while conventional linearly polarizing elements are useful in reducing glare associated with reflected light, they are not, however, well suited for use under certain low-light conditions, because of the static coloring agent. In addition, because conventional linearly polarizing elements have only a single, tinted linearly polarizing state, they are limited in their ability to store or display information.
As discussed above, conventional linearly polarizing elements are typically formed using sheets of orientated polymer films containing a dichroic material. Thus, while dichroic materials are capable of selectively absorbing one of two orthogonal plane polarized components of transmitted radiation, if the molecules of the dichroic material are not suitably positioned or aligned, no net linear polarization of transmitted radiation will be achieved. Due to the random positioning of the molecules of the dichroic material, selective absorption by the individual molecules will cancel each other such that no net or overall linear polarizing effect is achieved. As such, suitable positioning of the molecules of the dichroic material is typically achieved by alignment thereof with another material, which results in a net linear polarization.
In contrast to the dichroic elements discussed above, conventional photochromic elements, such as photochromic lenses that are formed using conventional thermally reversible photochromic materials, are generally capable of converting from a first state, for example, a “clear state,” to a second state, for example, a “colored state,” in response to exposure to actinic radiation, and then reverting back to the first state in response to, actinic radiation, such as the absence or reduction of exposure to actinic radiation, and/or thermal energy. As such, conventional photochromic elements are generally well suited for use in both low-light conditions and bright conditions. Conventional photochromic elements, however, that do not include linearly polarizing filters are generally not adapted to linearly polarize radiation. That is, the absorption ratio of conventional photochromic elements, in either state (e.g., clear state and/or colored state), is generally less than two. As used herein, the term “absorption ratio” refers to the ratio of absorbance of radiation linearly polarized in a first plane to the absorbance of the same wavelength radiation linearly polarized in a plane orthogonal to the first plane, in which the first plane is defined as the plane with the highest absorbance. Therefore, conventional photochromic elements are not capable of reducing glare associated with reflected light to the same extent as conventional linearly polarizing elements. To address this deficiency, photochromic-dichroic materials have been developed. Photochromic-dichroic materials provide both photochromic properties (i.e., having an absorption spectrum for at least visible radiation that varies in response to at least actinic radiation), and dichroic properties (i.e., capable of absorbing one of two orthogonal plane polarized components of at least transmitted radiation more strongly than the other).
Photochromic materials and photochromic-dichroic materials can be incorporated into a substrate or an organic material, for example a polymer substrate, including liquid crystal polymer substrates. When photochromic materials and photochromic-dichroic materials undergo a change from one state to another (e.g., from a clear state to a colored state), the molecule(s) of the photochromic compound or photochromic-dichroic compound typically undergo a conformational change from a first conformational state to a second conformational state. This conformational change can result in a change in the amount of physical space that the compound occupies. For certain photochromic materials and certain photochromic-dichroic materials, however, to effectively transition from one state to another state (e.g., to transition from a clear state to a colored state, or to transition from a colored state to a clear state, and/or to transition from a non-polarized state to a polarized state, or to transition from a polarized state to a non-polarized state) the photochromic compound or photochromic-dichroic compound typically requires a chemical environment that is sufficiently flexible to allow the compound to transition from a first conformational state to a second conformational state at a rate that is at least sufficient to provide the desired response on over an acceptable time frame. Liquid crystal polymers can provide such a sufficiently flexible environment.
Organic materials, such as polymers and/or liquid crystal polymers, typically include stabilizers, such as thermal stabilizer and/or ultraviolet light stabilizers, to limit and/or delay degradation of the organic material due to exposure to elevated temperatures and/or ultraviolet light. The presence of stabilizers in organic materials containing dichroic materials, such as photochromic-dichroic materials, can disrupt alignment of the dichroic materials, resulting in an undesirable reduction in absorption ratio values. Alternatively or additionally, when the organic material is composed of or contains liquid crystal materials, such as liquid crystal polymers, the presence of stabilizers can undesirably disrupt alignment of the liquid crystal materials. Still further alternatively or additionally, to disrupting liquid crystal alignment, the stabilizers may not be sufficiently soluble in the liquid crystal material, such as a liquid crystal polymer matrix, resulting in an undesirable reduction in clarity (e.g., an increase in haze) of the material.
It would be desirable to develop new stabilizers that can be used in compositions containing liquid crystal materials. In addition, it would be desirable that such newly developed stabilizers minimize or result in no disruption of liquid crystal alignment and/or have improved solubility in compositions containing liquid crystal materials. It would be further desirable that such newly developed stabilizers enhance liquid crystal alignment in compositions containing liquid crystal materials.